The present invention relates to a joint compound composition for building elements, particularly paper-faced plasterboards, and to a method of producing a structure such as a partition, a wall trim or a ceiling.
It is well known to use building panels such as plasterboards for producing partitions and trims for vertical or inclined elements or for producing suspended or non-suspended ceilings.
These plasterboards are generally formed from a core, essentially made of plaster, covered on each of its faces with a sheet serving both as reinforcement and as facing, which sheet may be formed from paper or from inorganic fibres.
European Patent Application published under the number 496 682 discloses a joint compound for filling the joints formed by the contiguous lateral edges of two plasterboards and for forming a tapeless joint. This joint compound comprises, in percentages by weight calculated with respect to all of the dry matter:                5 to 30% of hemihydrate plaster;        7 to 25% of polymer as a suspension in an aqueous phase and/or as a redispersible powder;        if necessary, other additives for adjusting the setting time and/or for adjusting the quality of the paste and/or the adhesion of the joint compound; and        at least one inner filler in an amount making up the composition to 100%.        
Example 15 of that patent application discloses in particular a composition comprising, in parts by weight:                194 parts of alpha-plaster;        108 parts of a vinyl acetate/ethylene copolymer stabilized with polyvinyl alcohol and having a solids content of 55%;        6.6 parts of a vinyl acetate/ethylene copolymer as a redispersible powder;        0.67 parts of cellulose ether;        669.3 parts of limestone        2.25 parts of polyvinyl alcohol;        0.027 parts of polyacrylamide;        2.25 parts of clay;        0.045 parts of white Portland cement;        1.12 parts of gypsum+starch (50/50); and        0.125 parts of hydrolytically degraded keratin.        
U.S. Pat. No. 5,653,797 discloses a composition for filling the joints formed by the adjacent edges of panels formed from plasterboards, this composition comprising:
(a) calcium sulphate hemihydrate;
(b) water in an amount sufficient to adjust the viscosity of the composition;
(c) a set retarder comprising a composition based on a polymer comprising acrylamide and acrylic acid monomers; and
(d) a set accelerator comprising a metal salt comprising acid cations.
That patent mentions the possible use of a binder such as a latex or polyvinyl alcohol.
Table II of column 8 of that patent indicates a composition comprising:                20 to 30% of calcium sulphate hemihydrate;        1 to 4% of a latex-type binder;        1 to 10% of perlite;        10 to 50% of limestone;        0.5 to 4% of clay;        0.1 to 1% of a thickener;        0.5 to 3% of talc;        1 to 4% of mica;        0.01 to 1% of the particular retarder described above;        0.01 to 2% of the particular accelerator described above;        0.01 to 1% of a preservative; and        20 to 30% of water.        
However, when this composition hardens, it undergoes excessive shrinkage.
International Application published under the number WO 99/48833 discloses a composition that can be used for filling joints between adjacent plasterboards. This composition causes hydraulic setting when it is mixed with an effective amount of a set accelerator and it comprises a material such as plaster and a particular retarder based on a sulphonated polymer or copolymer.
Example 11 of that application discloses a composition for filling joints having the following formulation (in % by weight):                35.9% of calcium sulphate hemihydrate;        4.1% of a vinyl acetate/ethylene copolymer, in 50% water;        7.1% of perlite;        0.3% of cellulose ether;        8.1% of limestone;        2.6% of wollastonite;        2.6% of kaolin;        0.1% of lime;        1.0% of retarder; and        38.3% of water.        
This composition also has the major drawback of resulting in excessive shrinkage.
The joint compounds or compositions that have been described have the drawback of being unsatisfactory because they do not have all the following qualities taken together:                good workability;        a colour compatible with that of the building elements for which they are intended; and        the ability to obtain a joint having zero or almost zero shrinkage and good mechanical properties (hardness, tensile strength, compressive strength, impact strength, and slight deformability or elasticity in order to prevent cracking).        
Furthermore, the compositions of the aforementioned documents U.S. Pat. No. 5,653,797 and WO 99/48833 have the particular feature of containing a powerful retarder that blocks the hydration of the calcium sulphate hemihydrate. It is therefore the user who adds an accelerator in order for this hydration to start. This entails difficulties when the joint compound is being used on a work site, since there is a risk of the user not adding the correct dose of accelerator.
In addition, these compositions change over time. Depending on the time that has elapsed since the preparation of the composition, the same amount of accelerator added at the moment of application does not have the same effects. It follows that there is uncertainty as regards the hydraulic setting in the sense that it is not possible to know precisely when this setting will take place.
Moreover, other problems arise when the building element is a plasterboard covered with a paper facing:                it is necessary, on the one hand, for the joint compound to have good adhesion to the paper facing and, on the other hand, for the adhesion of a wallpaper to the joint to be also good but not excessive, so as to make it possible to strip wallpaper one or more times without either tearing or degrading the joint compound; and        the joint compound must have a water and/or paint absorption as close as possible to that of the facing paper, so as to avoid having to apply a coat of primer before wallpapering or painting.        
The aim of the invention is therefore to provide a tapeless joint compound composition which, after mixing with water, gives a joint compound that has the abovementioned qualities and characteristics. It is based on the discovery that, in order for a joint compound to be satisfactory, it is necessary for it to contain calcium sulphate, a binder and a water repellent, and to do so in well-defined proportions.
Thus, the subject of the invention is a joint compound composition comprising:
a) 50 to 60% of calcium sulphate hemihydrate;
b) 5 to 15% of an organic binder in powder form; and
c) 0.05 to 0.2% of a water repellent.
Such a composition, after mixing with water, therefore gives a hydraulic setting joint compound that can be used by itself, that is to say it is both a filling joint compound and a finishing joint compound, and makes it possible to produce a joint without having to use a tape.
The subject of the invention is also a method of producing a structure, in which:                building elements are juxtaposed;        the joint compound composition according to the invention is mixed with water so as to obtain a joint compound;        the space between the building elements is filled by means of the joint compound obtained above, without using a tape; and        the joint compound is left to harden.        
Finally, the invention relates to a joint as obtained by implementing this method.
Other features and advantages of the invention will become apparent on reading the following description.